A demand oxygen system for generating overpressure in the interior space of a mask has become known from EP-B 91 843. The prior-art demand oxygen system is provided with an adapter connection piece, which can be attached to a connection piece of a breathing mask. The demand oxygen system consists of a switching member for generating the overpressure, which switching member is connected to a sensor, which acts as an actuating element and triggers the switchover to overpressure respiration when the adapter piece with the sensor is inserted into the connection piece.
It is disadvantageous in the prior-art demand oxygen system that the switchover to overpressure respiration is performed regardless of whether the user of the mask has put on the breathing mask or not. If the demand oxygen system is inserted into the connection piece and switched to overpressure operation when the breathing mask has not been put on, pressurized gas escapes continuously via the metering valve of the demand oxygen system into the environment, which may lead to premature depletion of the respiration gas reserve.
A mouthpiece valve for gas masks and breathing equipment, which consists of a valve housing and a tube displaceable in the valve housing as an actuating element, has become known from DE 28 42 247. When the mouthpiece located at the valve housing is put on, the tube is displaced in the direction of the mouthpiece, as a result of which the passage for the respiration gas is released. The prior-art mouthpiece valve is used only to interrupt the respiration gas supply when the mouthpiece has been removed. It is disadvantageous in the prior-art mouthpiece valve that the tube always must be held against the full force of a restoring spring in the open position of the valve.
A lung-controlled diaphragm valve for breathing masks with a switching member in the form of a rocker arm, which is pretensioned by spring elements and can be brought from a stable switched-off position into the operating position for the overpressure respiration by pressure on a carrier acting as an actuating element, has become known from DE-C 35 39 668.
It is disadvantageous in the prior-art diaphragm valve that the switchover from the switched-off position to the operating position can be performed regardless of whether the breathing mask has been put on or not, and it can happen that the user of the mask neglects to bring the rocker arm into the switched-off position. Respiration gas is lost unused in this case.